This proposal aims to extend our behavioral studies of dominance status in crayfish to a search for neurochemical correlates. Hierarchical structures are remarkably similar in social groups ranging from molluscs to humans with individuals embedded in an often linear arrangement of social ranks. Individual characteristics such as size or stamina define the initial states but final ranks are acquired and reinforced through self- structuring processes. Changes in behavior that result from wins or losses play an essential role where winning encourages further success and losses lead to further losses. A behavioral characterization in systems ranging from social insects to children at day-care centers has lead to a new understanding of the fundamental principles that organize the acquisition and defense of social status. The proximate mechanisms of these winner/loser effects, however, are neither well understood nor intuitively obvious. Instances of neurochemical plasticity have been reported in the context of aggressive state, dominance, winning, or social status, but a consistent picture has yet to emerge with regard to the main substances of interest, the sign and magnitude of such effects, or their time course. Crayfish hierarchies, a naturally simple model system, are uniquely suited for such work: status is determined by dyadic and directly observable, stereotyped behaviors; aggressive state and physical superiority are the main determinants of dominance; no coalitions exist; and strong winner and loser effects lead to linear hierarchies via self-structuring. Our current work on the behavioral effects of wins and losses generates detailed behavioral profiles for several hundred individuals, including estimates of initial aggressive state, the types of fighting strategies used, readiness for retreat, and changes in all of these over time as dominance develops. Neurochemical feed-back loops involving amines and steroids are likely candidates for the behavioral re-enforcement mechanisms and determinants of rank. We can now apply our expertise in the analyses of dynamic phenomena to the functioning of neuromodulatory and neurohormonal systems in individuals with known behavior. Specifically, we will evaluate whether changes in behavior are accompanied by shifts in neurochemical function. Our main effort will focus on amines and ecdysteroids in the CNS or hemolymph, and we plan to develop the groundwork for a future R01 focusing on changes in gene expression of key enzymes and receptors. Multiple levels of organization are readily accessible in this model system. It thus offers unique opportunities for exploring the nature of neurochemical changes at a level of analysis that is difficult to achieve in more complex mammalian systems. Results from this work will allow us to refine and focus the search for neurochemical correlates of social status in vertebrates.